1. Field of the Invention
Copying machines, information processing apparatuses, such as word processors or computers, and communication apparatuses have been extensively used in recent years. As output devices for forming (recording) images from data output by any of these apparatus, output devices which perform digital image recording employing an ink jet recording head have been available on the market. Further, high-quality or color information apparatuses or communication apparatuses have also been available, and hence there has been an increasing demand for high-quality or color recording devices.
To improve the recording speed, the ink jet recording apparatus employs a recording head (hereinafter referred to as a multihead) in which a plurality of recording elements, ink discharge ports and liquid flow passages are arrayed at a high density. Color ink jet recording apparatuses having a plurality of multiheads for cyan, magenta, yellow and black colors are also known. In various other types of recording apparatuses, such as thermal recording apparatuses employing an ink ribbon or thermosensitive recording apparatuses for performing recording using the thermosensitive paper, a multihead in which a plurality of recording elements are arrayed at a high density is employed to improve the recording speed, as in the case of the ink jet recording apparatus.
Ink jet recording apparatuses are classified into those which employ electrothermal transducers and those which employ electromechanical transducers, such as piezoelectric elements. In the former type, a heating element (an electrothermal transducer) is provided near a discharge port. Ink is heated locally and changes in the pressure of the ink are generated by application of an electrical signal to that heating element, resulting in discharge of the ink from the discharge port. Ink jet recording apparatuses of the type which employ, as means for discharging ink, electricity-pressure transducers for applying a mechanical pressure to the ink to discharge it, such as piezo elements, are also known.
As the method of performing recording control required for displaying gray scale images in the above-described types of ink jet recording methods, there are the dot density control method in which a number of recording dots having a fixed size per unit area is controlled, and the clot size control method in which the size of a recording dot is controlled. The latter dot size control method requires complicated control to finely change the size of a recording dot, and the region in which the recording dot size can be controlled is limited. Thus, the dot density control method in which recording of gray scale images is performed by controlling the number of recording dots per unit area is normally adopted.
There are two typical binarization techniques required for displaying gray scale images in the above-described dot density control method, ordered dither method and conditionally determined dither method.
The ordered dither method employs, as a dither matrix, a matrix in which thresholds having no relation to input pixels are distributed orderly. The input image is binarized on the basis of the pixel position and the thresholds on the dither matrix by repeating the dither matrix in the horizontal and vertical directions. Accordingly, in this method, the number of gray levels is limited by the matrix size. That is, to increase the number of gray levels, the matrix size must be increased. However, an increase in the matrix size increases the size of a single pixel of a recorded image constituted by a single matrix, thus deteriorating resolution of the recorded image.
The conditionally determined dither method, such as the error diffusion method, is a method in which the value of an input pixel or the threshold is varied with the pixels surrounding the input pixel taken into consideration. The conditionally determined dither method has advantages in that it exhibits excellent compatibility between the gray scale and resolution and in that generation of a moire pattern in a recorded image obtained when the original image is a printed image is very low. However, a bright portion of the image readily becomes irregular, deteriorating the image quality.
In the above-described binarization methods, there is a difference in the image quality. The processing speed also differs between the above two binarization methods. Whereas each pixel is independently processed in the ordered dither method on the basis of the relation between the value of the input pixel and the threshold, not only the relation between the input pixel and the threshold but also the values surrounding the objective pixel must be referred to in the conditionally determined dither method because the input pixel or the threshold is determined by the values surrounding the objective pixel. Thus, the processing speed of the binarization technique in the ordered dither method is higher than that in the conditionally determined dither method.
Regarding the image quality and processing speed, printers in which a printer driver is designed to select either of the two binarization methods depending on an output image or usage of the image are known.
2. Description of the Related Art
The regulated array of thresholds (hereinafter referred to as a gray scale pattern) of the generally employed ordered dither method falls into dot dispersion type and dot concentration type. FIG. 2A illustrates a typical dot dispersion type pattern which represents 64 gray levels and FIG. 2B illustrates a typical dot concentration type pattern which represents 64 gray levels. With these patterns, recording is performed on the dots having up to a number corresponding to the gray level value indicated by an image.
FIG. 2A illustrates a dot dispersion type pattern called Bayer type. With the dot dispersion type gray scale pattern shown in FIG. 2A, recording of dots can be conducted over an entire area of the unit area. In the ink jet recording apparatus, since the ink spreads on the recording medium, making the actually recorded dots wider than desired recording dots, the dot dispersion type gray scale pattern offers the gray scale characteristics shown in FIG. 3A. Consequently, in a high density portion of an image in which many dots are recorded, gray levels do not linearly respond to the numbers of recorded dots, making gray scale display difficult. The above-described gray scale characteristics can be corrected such that a gray level responds almost linearly to the density of an image. However, this creases a pattern which is used over several gray levels or a pattern which is not used at all. In the ordered dither method, since the number of gray levels is determined by the matrix size, the above gray level correction further reduces the number of gray levels, making continuous gray scale display difficult.
FIG. 2B illustrates a dot concentration type gray scale pattern called Fatting type pattern. With the dot concentration type gray scale pattern shown in FIG. 2B, the number of recorded dots increases gradually starting from almost the center of the matrix. Therefore, the amount of dots to be recorded almost linearly corresponds to the proportion of the actually recorded area of the dots relative to the entire area. The gray scale pattern shown in FIG. 2B has gray scale characteristics shown in FIG. 3B due to the relation between the amount of dots recorded in the gray scale pattern area and the proportion of the dot recorded area. Thus, continuity of the gray scale is higher than that of the dot dispersion type. However, since the recorded dots concentrate on almost the center of the matrix, non-recorded areas are generated, making it impossible for the printer to exhibit its own resolution.
Where the number of nozzles of a multinozzle is a multiple of the vertical length of the gray scale pattern, the frequency with which each nozzle is used greatly differs for every nozzle particularly with the dot concentration type gray scale pattern. Accordingly, the often used nozzles deteriorate, and wettability between the nozzle constituting material and the ink changes, making the amount of or direction of ink discharged different for every nozzle. Consequently, the regular array of dots in the ordered dither method becomes irregular. This irregularity of the dot array makes images irregular and reduces the lifetime of the head.
In view of the aforementioned problems of the prior art, a primary object of the present invention is to provide a gray scale pattern which enables irregularities on an image, caused by differences between the frequencies with which the individual nozzles are used, to be reduced and which enables the lifetime of a recording head to be increased, as well as an ink jet recording apparatus which is capable of recording with such a gray scale pattern.
Another object of the present invention is to provide a gray scale pattern which enables gray scale characteristics to be improved and which enables irregularities on an image, caused by misregistration, to be reduced, as well as an ink jet recording apparatus which is capable of recording with such a gray scale pattern.
To achieve the above-described objects, the present invention provides a recording apparatus which comprises a recording head having an array of a plurality of recording elements for performing recording on a recording medium, and gray scale control means for performing multilevel recording by controlling, according to a predetermined gray scale pattern, a number of dots to be recorded in a single pixel formed of a plurality of recording dots formed by the recording elements of the recording head.
The predetermined gray scale pattern includes a plurality of blocks each having an area smaller than an area of the single pixel and in which dots to be recorded in the block increase starting from almost a center thereof as the number of dots recorded in the single pixel increases, and is designed such that as the number of dots recorded in the single pixel increases, the number of dots to be recorded in the block increases by a predetermined amount in a predetermined order of the plurality of blocks.
The present invention further provides a gray scale pattern for use in multilevel recording performed by a recording apparatus including a recording head having an array of a plurality of recording elements for performing recording on a recording medium, and gray scale control means for performing multilevel recording by controlling a number of dots to be recorded in a single pixel formed of a plurality of recording dots formed by the recording elements of the recording head. The gray scale pattern includes a plurality of blocks each having an area smaller than an area of the single pixel and in which dots to be recorded in the block increase starting from almost a center thereof as the number of dots recorded in the single pixel increases. As the number of dots recorded in the single pixel increases, the number of dots to be recorded in the block increases by a predetermined amount in a predetermined order of the plurality of blocks.
The present invention further provides a recording apparatus for performing recording by a recording head having a plurality of recording elements, which comprises gray scale means for performing multilevel recording by a number of recording dots in a single pixel formed of a plurality of dots formed by the recording elements of the recording head, and a gray scale pattern set according to each of gray levels in the single pixel. The maximum difference between numbers of times the individual recording elements are used in a range defined by the gray scale pattern is not more than twice.
The present invention is designed to make uniform the frequencies with which individual recording elements of a multihead are used in a recording apparatus for performing gray scale recording by controlling a number of dots recorded in a unit area. The gray scale pattern used to display gray scale is a pattern in which a plurality of blocks each comprising a dot concentration type gray scale pattern having a size smaller than that of the gray scale pattern is dispersed. The maximum difference between numbers of times the recording elements in the range of the gray scale pattern are used can be restricted within twice, and individual nozzles can be used uniformly.
Consequently, imbalance of changes in the recording elements with time, caused by the differences between the frequencies with which the recording elements are used, can be restricted, thus restricting irregularities in an image.